Bipolar membranes are known to be useful for their ability to rectify alternating current, to improve desalination processes, to act as biological membranes, and to produce acids and bases from salts. This latter property has great potential usefulness, since it allows the production of commercially important acids and bases from readily available neutral salts, e.g., the formation of HCl and NaOH from table salt. The potential for carrying out this type of conversion by means of bipolar membranes, a process known as electrodialytic water-splitting, has not yet become an industrial process, primarily because of deficiencies in the properties of the bipolar membranes themselves and the difficulty of fabricating bipolar membranes having reproducible properties on a large scale.
Many bipolar membranes prepared by various procedures have been reported in the literature. Initially, bipolar membranes were prepared by bonding together polymer layers containing ion-exchange resin particles of opposite charges, as described for example in U.S. Pat. No. 2,829,095. The fusion of conventional anion and cation transfer membranes (described for example in U.S. Pat. Nos. 2,731,411 and 2,860,097) by heat and pressure to yield bipolar membranes was also reported, as in U.S. Pat. No. 3,372,101 and British Pat. No. 1,038,777. Preparation of bipolar membranes from a single sheet of polymeric material by selective functionalization of one side to give anion or cation permeability and the other side permeability to ions of the opposite charge was described in U.S. Pat. Nos. 3,388,080 and 3,654,125, as well as later U.S. Pat. Nos. 4,024,043, 4,057,481 and 4,140,815. Although the bipolar membranes prepared by these later methods have the desired properties of
(i) Good mechanical strength,
(ii) High permselectivity,
(iii) Low membrane voltages,
(iv) Stability, and
(v) Operability at high current density,
quality control is poor, particularly on a large scale, primarily due to the difficulty in controlling the diffusion into and reaction of reagents with the film. While such diffusion and reaction control is possible with small, carefully selected uniform film, the non-uniformity inherent in large films and the strict requirements for diffusion and reaction control make large-scale manufacturing of the membranes quite difficult.
Another approach to making high-performance bipolar membranes with the desired properties listed above may be found in U.S. Pat. No. 4,116,889. Unfortunately, fabrication of such membranes has proven difficult because the formation of a uniform, fault-free, anion permeable layer by solution casting of a polymer system which is undergoing gelation requires careful control of the physical properties of the polymer, the mixing conditions, the casting process, and the gelation and drying times. Deviations from these controls may lead to permanent scratches by the casting knife, non-uniform thickness and composition of the layer, and cracking. Accordingly, the fabrication of bipolar membranes which depends on the control of the formation of homogeneous polymer gels is difficult to practice where large-scale manufacturing is contemplated.
In summary, the prior art has not been able to fabricate a high-performance bipolar membrane, one which possesses the desired properties listed above, which can be prepared by a straightforward and reliable procedure.